1. Field of the Invention
The present invention relates generally to a data extracting circuit, and is directed more particularly to a data extracting circuit which extracts data of a binary digit signal or plural digit signals.
2. Description of the Prior Art
In general, when a binary digit input signal is constant in amplitude and is not bias-fluctuated, the data "1" and "0" of the input signal can be easily and correctly extracted by comparing the level of the input signal with a threshold voltage of a constant level.
However, in the case where the envelope of an input signal is varied by noise and so on, the data "1" and "0" of the input signal can not be extracted correctly by comparing the level of the input signal with the constant level threshold voltage.
Therefore, the following data extracting circuit may be considered. That is, in the data extracting circuit an input signal is applied to both a positive peak hold circuit and to a negative peak hold circuit to provide positive and negative peak-held voltages. The positive and negative peak-held voltages are added at the rate of 1:1 to provide a mean voltage thereof, and then the input signal is compared with the mean voltage which serves as a threshold voltage by a level comparator which delivers an output signal of the data "1" and "0" of the input signal or extracts the data "1" and "0" from the input signal.
According to the above data extracting circuit, when the varying frequency of the envelope of the input signal is sufficiently low as compared with the frequency of the input signal, the change point of the data of the input signal from "1" to "0" can be correctly detected without being affected by the envelope variation of the input signal. However, according to this data extracting circuit, the threshold voltage for detecting the data changing point from "1" to "0" of the input signal is also provided with the peak voltage of the input signal prior to the data changing point. Therefore, if noise with a frequency close to that of the input signal is contained therein and, the input is a signal reproduced from magnetic tape on which a signal was recorded with a prior recorded signal thereon which was not erased or signals are recorded, in a so-called superimposed manner, the varying frequency of the envelope of the input signal will be near the frequency of the input signal and, the input signal will drift between the peak point and the changing point and the threshold voltage will be shifted by that amount with the result that the change point can not be correctly detected.